In wireless communication systems in particular, communication quality and communication capacity often have an inverse relationship. For example, as communication capacity is increased, such as through more dense reuse of traffic channels, signal quality may be decreased, such as through each such traffic channel experiencing higher levels of interference energy. Accordingly, wireless communication service providers must often balance providing desired levels of communication capacity with service quality issues.
In code division multiple access (CDMA) networks, for example, a number of communication signals are allowed to operate over the same frequency band simultaneously. Each subscriber unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the subscriber unit. The subscriber units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular subscriber unit. However, despreading of the desired subscriber unit's signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other subscriber units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of subscriber units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver.
It is therefore desirable to control the amount of energy radiated within a particular service area to thereby reduce interfering energy experienced by subscriber units operating therein. For example, in the aforementioned CDMA networks, transmitted signals are often power controlled to reduce energy transmitted within the CDMA frequency band while maintaining sufficient power to provide an acceptable signal at a receiving unit. Through intelligent power control, excess energy within the service area may be limited and, therefore, signal quality improved and/or capacity increased.
Further capacity and/or signal quality improvement may be provided in communication systems through the use of directional antenna beams in the communication links, such as may be provided using “smart antenna” systems. Adaptive array antennas may be utilized to provide enhanced signal quality through advanced “beam forming” techniques as shown and described in the above referenced patent application entitled “Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement.” For example, angle of arrival (AOA) information determined from a received signal at an adaptive array antenna may be utilized in determining beam forming coefficients for use in providing narrow beams in the reverse link in order to provide improved capacity.
Additionally, direction estimation based on AOA information from the reverse link might be used in providing narrow beams in the forward link. However, the use of such directional antenna beams in the forward link often does not provide the desired containment of the radiated energy and/or does not provide a desired signal quality at a target remote subscriber unit. Generally, the signal of such a narrow antenna beam gets spread over an area different than that of the formed radiation pattern, such as due to the effects of scatters disposed within the radiation pattern area. Accordingly, if a forward link beam is used to simply direct the power in the estimated direction of a target subscriber unit, a significant portion of the transmitted power will not reach the target subscriber unit but will instead be spread across an area wider than that required to maintain the desired communication link. This both results in a waste of the transmitted power as well as increased interference energy experienced at other ones of the subscriber units.
Increased interference energy experienced at the subscriber units results in decreased capacity, whether the decreased capacity is a result of fewer subscriber units being accommodated, the subscriber units being accommodated being provided decreased throughput, or a combination thereof. This undesired result may be further aggravated as a result of the interference energy causing the interfered subscriber units to increase power associated with their communications and, thus, the system experiences a related increase in the associated interference energy.
Moreover, the use of such narrow beams in providing communications links may introduce unique problems associated with their implementation. For example, cellular or personal communication services (PCS) systems using CDMA communication techniques often utilize both a pilot signal and a traffic signal to establish communications. The pilot signal generally provides a known signal and is used by receiving devices in demodulating a traffic signal. In the forward link, i.e., the base station to subscriber unit link, a common pilot signal is often used for multiple subscriber units, such as all subscriber units in a cell or a sector. Accordingly, it is typically desirable to provide this pilot signal throughout an area in which multiple subscriber units are likely to be located.
The use of narrow beams for reducing radiated and/or accepted energy as discussed above can be problematic with respect to use of such a pilot signal. For example, if the pilot signal were to be transmitted in a narrow beam corresponding to the traffic signal of a particular subscriber unit, other ones of the plurality of subscriber units may not receive the pilot signal for use in demodulating their corresponding traffic signal. Accordingly, it is often desirable to provide the pilot signal in an area larger than that of the narrow beam associated with a particular subscriber unit. However, this often results in a phase mismatch problem at one or more of the subscriber units. Specifically, as the link channel associated with the pilot signal (e.g. wide beam) is not the same as that of the link channel associated with the traffic signal (e.g. narrow beam), the phase information extracted from the pilot signal may no longer accurately correlate with the traffic signal as received by a subscriber unit. Although perhaps providing phase matching to within acceptable limits for lower order modulation schemes, such as BPSK, such a phase mismatch is likely to result in unacceptable communication errors, such as an excessive bit error rate (BER), in higher order modulation schemes, such as QPSK, 8PSK, etcetera.
Accordingly, a need exists in the art for systems and methods which provide for the use of optimized beams, such as beams having a minimized or otherwise reduced beam width, to thereby control the amount of interference energy radiated and/or accepted. Moreover, a need exists in the art for the use of such optimized beams to provide a desired and predictable signal phase relationship, such as with respect to a demodulation reference signal.